This invention can be used in digital transmission applications and, more particularly, in the case where, after having transmitted an electromagnetic wave by modulation, information is restored or retrieved by coherent demodulation.
The range of application of the invention is thus extremely wide and covers data transmission modems, hertzien beams, space communications systems, indeed optics (in the case of heterodyne links).
The demodulation of a wave assumes that the receiver receiving the transmission knows the frequency and phase of the transmission carrier wave.
To this end, a demodulation wave is generated by a voltage-controllable oscillator (OCT), the control voltage of this oscillator being supplied by a phase comparator delivering an error voltage according to the phase deviation .PHI. between the modulated carrier wave and the wave generated by the oscillator.
Having regard to the fact that the transmission carrier wave may be amplitude and/or phase modulated, the structure of the phase comparator depends on the modulation used.
The recovery of the carrier wave by the phase locking loop firstly requires that the oscillator has a frequency close to the received modulated wave. As this is effected by phase comparison between the received modulated wave and the wave delivered by the oscillator, the frequency from the wave of the oscillator is modified continuously until it reaches the frequency of the modulated wave.
This frequency hooking or acquisition phase is not guaranteed in those devices traditionally used to recover the carrier wave of a digitally modulated signal.
In fact, dummy frequency acquisitions do occur and particularly when the difference between the bearer frequency and the frequency of the local receiving oscillator is a whole multiple of half the modulation speed.
This phenomenon is normal when the digital transmission speeds are slow and when the drift of the oscillators, associated with a possible Doppler effect (in the case of satellite transmissions), requires a large acquisition range. For example, this applies to the transmission system of the IMMARSAT STANDARD C type.
A device for the coherent demodulation of signals modulated in two phase states, commonly known as a "Costas Loop", is shown on FIG. 1.
This device does not make it possible to resolve the problems of dummy frequency acquisitions in cases of two-state phase modulation. However, it is possible to elementarily detect a dummy frequency acquisition and relaunch acquisition by specially forcing the control signal of the oscillator, as indicated in the description given hereafter.
The device includes an voltage-controllable oscillator 4 receiving on one control input 27 an error signal .epsilon.(.PHI.) and having one output 26 delivering a wave whose phase depends on the error signal.
The device also includes two demodulators 1 and 2. The first one has two inputs 20 and 21 receiving respectively the modulated wave and the wave delivered by the voltage-controlled oscillator, and has one output 22 delivering a signal X1. The second one has two inputs 23 and 24 respectively receiving the modulated wave and the wave delivered by the voltage-controllable oscillator, firstly dephased by 90.degree. by a static phase shifter 3, and one output 22 delivering a signal Y1.
A lowpass filter 5 is disposed at the output of the demodulator 1. This filter delivers a filtered signal P(t). Similarly, a lowpass filter 6 is disposed at the output of the demodulator 2, this filter delivering a filtered signal Q(t). These filters are required to suppress the harmonics at the output of the demodulator. They may also be used to maximize the signal/noise ratio if this has not been done with the aid of the bandpass filter placed upstream of the two demodulators.
The device of FIG. 1 also includes a loop filter 7 which delivers at its output the error signal .epsilon.(.PHI.), this filter receiving at its input the output signal of a multiplier 8.
The filtered signals P(t) and Q(t) are applied to the two inputs of a multiplier 8.
The unit thus forms a phase locking loop.
The device also includes a threshold detector 9 followed by a decision device 10 controlled by a clock (not shown).
This unit makes it possible to known the level of the output signal of the lowpass filter and thus to know whether or not the phase locking is correct. In fact, when a locking is correct, one of the two signals is virtually nil, whereas in the presence of a dummy acquisition, the signals of the quadrature and phase signals have identical dynamics.
Unfortunately, this simple system has the drawback of being unable to be extended to modulations in a larger number of states and in particular gives no indication on frequency shift, that is concerning the direction in which the local voltage control oscillator needs to be orientated so as to reduce this frequency shift.
However, this problem may be resolved by adding at the input of the local oscillator a periodic low frequency signal, known as a scanning signal, which is inhibited as soon as the above-mentioned criterion is verified.
Unfortunately, this solution may require a complete period of the scanning signal which is, as mentioned above, a low frequency signal, which means that the acquisition time may remain long.
However, the Applicant has noticed that this time could be substantially reduced provided it is possible to estimate the direction in which the voltage-controlled oscillator needs to be orientated.
A second method consists of observing that in the presence of a dummy acquisition, the loop noise is greater when the system is correctly locked. But then the decision criterion is less reliable, as a large loop noise may simply be linked to poor propagation conditions and thus a large loop noise rate does not necessarily indicate a dummy frequency acquisition. This method is accordingly very much open to criticism.
Furthermore, the French patent application filed on Oct. 4, 1983 and registered under the number 2552959 concerns a device for recovering a carrier wave with the aid of a Costas demodulator to which a phase and frequency comparator is added for signals modulated in N phase states.
The principle of this device consists of carrying out a general processing on the signals P(t) and Q(t) of the quadrature and phase channels of the Costas demodulator so as to firstly draw up the error voltage of the servo system, and secondly a proportional voltage after filtering at the frequency shift. This technique makes it possible to considerably reduce acquisition time more effectively than with conventional systems using a blind scanning signal.
However, this device does not in itself resolve the problem of dummy frequency acquisitions particularly critical in IMMARSAT STANDARD C type low speed digital transmission systems, for example. In fact, in this type of transmission, dummy frequency acquisitions may occur when the difference between the bearer frequency and the frequency of the local oscillator is a whole multiple of half the modulation speed R (R=1/T).